The so-called CVD method where a solid material is formed from gaseous reaction materials has been normally used for many years to separate TiC or TiN by introducing titanium chloride and hydrogen and methane or nitrogen as carbon or nitrogen supply. It is similarly known to use aluminum chloride and carbon dioxide as donators for separating out Al.sub.2 O.sub.3. The process parameters in a CVD method can be set so that no solid adhering deposit, only loose deposits (powder) are formed. The CVD method is carried out in heated reaction chambers partially under subatmospheric pressure. According to the concentration of the reaction partners in the vehicle gas as well as the temperature and flow velocity there are various deposition rates, compositions, and structures or textures of the layer.
It is particularly important to the plasma CVD method that one can operate at low temperatures. If the reaction gas is mixed in a low-pressure glow discharge with an unbalanced plasma, the charge carriers in the gas are accelerated by the existing electrical field. The free travel between two impacts is dependent on the particle density or the pressure. If the particle energy reaches the applied voltage, molecules or atoms can be excited to the point of disassociation or ionization making possible chemical reactions that could otherwise only take place at higher temperatures. In principle low-pressure plasmas are created by applying a constant direct-current voltage or a pulsed direct-current voltage to a cathode-polarized workpiece as for example described in German 3,841,731.
In U.S. Pat. No. 4,909,183 at first an evacuatable chamber is described in whose center is arranged a rotatable holder for the coating substrate. The substrate itself is surrounded by a cylindrical cathodic electrode which is connected with a high-frequency voltage source. The substrate serves as anode so that when voltage is applied to the cathode a low-pressure plasma is created. Disadvantageously the material deposits on the cathode with the gas outlet apertures so that after a short time the gas flow is accurately eliminated.
U.S. Pat. No. 4,991,542 describes a reaction chamber in whose center is arranged a substrate body connected with a high-frequency sources and to whose sides there are diametrally opposite electrodes also serving as gas infeeds. The electrodes are formed as funnels and have multiple gas outlet apertures directed like a shower toward the substrate. Instead of connecting the voltage source with the substrate or with the substrate holder, it is connected with mesh electrodes which are arranged on both sides between the substrate and the gas-shower electrodes.
In European 0,257,620 a method and apparatus for forming a layer by a plasma-chemical process is described where at least three electrodes, preferably connected to a high-frequency source, are used which are arranged in a reaction chamber that is evacuatable and chargeable with gas. The substrate surface should extend perfectly or nearly perfectly parallel to an electrical field formed between the electrodes. The gas necessary to form the layer should flow between the and/or through the electrodes directly on the surfaces. The potential of succeeding electrodes should increase regularly. The electrodes build a containment which corresponds to the path of the electrical field formed between the electrodes. The electrodes can be formed as hollow cylinders and have outlet apertures for the gas. It is also possible to use annular electrodes which are arranged coaxial to the substrate having the surfaces like cylinders or electrodes that are arranged on the periphery of a cylinder.
Devices for plasma CVD coating are known from EP 0,503,820 which have multiple hollow cylindrical gas infeeds that are polarized as cathodes and have an inside diameter of about 1.9 cm and a length of 38.8 cm. The hollow gas infeed cylinders have several rows of outlet apertures each with six individual aligned openings. Data about the size of the openings is not given.
The inflow velocity gradient for the reaction gas should be according to the method described in EP 0,402,675 greater than 1050 cm/(cm.sec).